Aortic valve replacement has changed considerably in the last decade. Previously, valve replacement required a major procedure with cardiopulmonary bypass, stopping of the heart, excision of the diseased valve and then suture implantation of a valve prosthesis at the site of the excised valve. The procedure was often difficult for patients and some older patients were too ill to undergo surgery.
This all changed when it was found that the old diseased valve could be left in place and a prosthetic valve could be implanted inside the diseased valve using a catheter procedure. There was no need for cardiopulmonary bypass, no need to stop the heart and no need to suture a valve in position. In many countries the percutaneous procedure has become the most common and preferred way to treat patients.
The valve implant procedure involves using catheters to implant one of a variety of prostheses inside the old diseased valve. In general, the prosthetic valves use leaflets fashioned from tissue taken from pigs or cows. The leaflets sit inside mounting structures or frames. Common structures to support the leaflets include stents (self-expanding type stents such as used by Medtronic, balloon expanding stents such as used by Edwards, and a number of other companies), activatable frames (Sadra, Boston Scientific) and even inflatable frames (Direct Flow). To implant these devices, the leaflets are mounted on a frame, collapsed in catheters and then introduced inside the aorta of the patient. The valves are positioned inside the diseased native leaflets and then deployed and expanded to replace the function of the native aortic valve.
Development of this procedure has been complex and is a remarkable tribute to the doctors, engineers and companies who have overcome so many obstacles. There is one particularly vexing problem that still remains. A considerable number of patients develop complete heart block after the procedure. Complete heart block can occur immediately or it can be delayed days or weeks. The atrium sets the rate of contraction for the normal heart. The rate signal that originates in the atrium passes into the ventricles through specialized muscular conduction or conductive tissue at the top of the interventricular septum—just a short distance below the aortic valve. From the top of the septum, the signal passes to both ventricles and the ventricles contract and eject blood to the circulation. If the conduction tissue at the top of the ventricular septum is damaged, the signal does not pass and the ventricles do not receive the signal to contract. This condition is called heart block or complete heart block. The patient's heart may then stop completely, or it may contract at a very slow rate that is not consistent with survival. The patient may die suddenly or become very ill. This event can happen unexpectedly and there is a lingering risk for development of heart block for a prolonged period after percutaneous valve implantation.
Heart block has been seen with all of the prostheses used to date. It appears that the frame for the valve impacts against the conduction tissue and after a variable period of time damages the tissue and the tissue ceases to conduct the signal to contract to the ventricles. Heart block then occurs.
Heart block can result in sudden death or a hemodynamic crisis. The risk of heart block requires prolonged monitoring because of the unpredictable nature of the event. The treatment for heart block is implantation of a pacemaker. While this is a common and quite benign procedure, the effectiveness of the heart's contraction with a pacemaker never reproduces the contraction that results from a healthy native conduction system. And pacemakers are expensive and require lifelong surveillance necessitating visits by the patients to ensure their device is functioning properly and that the battery is still effective.
The rate of heart block that has been observed ranges from about 10% to as high as 30%. Despite the fact that almost a decade of work has been conducted, no valve and no procedure to date has been shown to eliminate the problem.
Considerable research has been conducted to understand this problem. Recently, interventional cardiologists have found that if the frame of the prosthetic valve sits less than 4 mm to 5 mm below the lowest point of the native valve, heart block almost never occurs. If the prosthesis sits lower than this the risk of heart block rises.
This makes good anatomic sense. Just beneath the aortic valve sits the membranous septum. The septum is a small region of non-muscular tissue that separates the two ventricles. It sits on the top of the interventricular septum. The conduction system that passes the signal to contract into the ventricle sits on the crest of the interventricular septum. The distance from the nadir of the aortic valve leaflets to the conduction tissue is approximately 4 mm. This corresponds exactly with the clinical observation by the interventional cardiologists.
The current trend is to make every effort possible to implant a prosthetic valve to ensure that its lowest point is positioned less than 4 mm below the nadir of the native aortic valve leaflets. This is no easy feat since the valves are introduced on long catheters passing from an entry point in the groin, up the aorta, around the aortic arch and then into the ventricle. The heart is beating and ejecting blood, and this makes accurate positioning difficult as well. It is extremely difficult to be sure that a valve will be deployed in the perfect position. The person performing the procedure is also concerned that if the valve sits too high, it may not engage inside the native leaflets and it may be ejected out of the correct position into the aorta.
It would be very useful to have devices, systems and methods to help the interventionist to place a prosthetic valve in the ideal position and/or otherwise reliably prevent damage to the conductive tissue. A goal should be to prevent force from being applied to the conductive tissue after implantation. And the prosthetic valve must not sit so high that it does not engage securely against the native leaflets and eject out of the correct position.